Entomofauna, ZEITSCHRIFT FÜR ENTOMOLOGIE VOL 0011-0377-0427

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Sntomojauna
ZEITSCHRIFT FÜR ENTOMOLOGIE

Band 11, Heft 23/1

ISSN 0250-4413

Ansfelden, 15.November 1990

The Ethology of the Solitary Bee
Andrena nycthemera Imhoff,1866
(Hymenoptera, Apoidea)
Klaus Schönitzer 6 Christine Klinksik
Zoologisches Institut der Universität München
Abstract
A large aggregation of nests of the solitary bee Andrena
nycthemera IMHOFF,l866,was investigated in southern Germany from 1983 to 1988 and in 1990. The nesting site is
a sandy slope with several hundreds of nests. Many bees
were labelled individually.
The following behavioral patterns of male Andrena nycthemera IMHOFF,l866, are described: crawling and inspecting holes, digging, aggressive behavior, patrolling
flights, territorial behavior, pouncing and mating. The
most important female behaviors described are: searching
for a nest site, repulse pouncing males, digging and
building nests, emerging from nests, sitting in the entrance, closing the nest entrance, orientation flights,
searching the entrance, provisioning, aggressive behavior (not yet described in Andrena females) and irregulär behavior at the end of the season. The females take

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care of usually one or two nests, up to four nests.Mating takes place on the surface of the soil at the nesting site.
During one season (1987) the nest aggregation was observed almost every day with suitable weather. For this
season the frequency of several behavioral patterns has
been compiled (Fig»9a, b) and its correlation with the
weather is discussed.
Sphecodes pellucidus SMITH,1845 (Apoidea, Halictidae)
and Leuoophora obtusa (ZETTERSTEDT, 1838) (Diptera, Anthomyiidae) are nest parasites of Andrena nycthemera IMHOFF,l866.
Zusammenfassung
Das Verhalten der Sandbiene Andrena nycthemera IMHOFF,
1866, wurde von 1983 bis 1988 und 1990 in einer großen
Nestaggregation in der Nähe von Dachau (Oberbayern) regelmäßig beobachtet. Der Nistplatz ist ein südexponierter Sandhang mit mehreren hundert Nestern. Viele Tiere
wurden individuell markiert.
Folgende Verhaltensweisen der Männchen von Andrena nycthemera IMHOFF,1866, wurden beobachtet: Krabbeln am Boden und Inspizieren von Löchern, Graben, agressives Verhalten, Schwarmflüge, Territorialverhalten, Kopulationsversuche, Paarung. Die wichtigsten beschriebenen Verhaltensweisen der Weibchen sind: Suchen nach einem Nistplatz, Abwehren von Männchen, Graben und Nestbau, aus
den Nestern kommen, im Eingang sitzen, den Eingang verschließen, Orientierungsflüge, Suchen nach dem Nesteingang, Pollen eintragen, aggressives Verhalten (war bisher für Andrena 99 unbekannt) sowie ungewöhnliche Verhaltensweisen am Ende der Saison. Die Weibchen versorgen
in der Regel ein oder zwei, maximal vier Nester. Kopulationen finden am Boden am Nestplatz statt.
In einer Saison (1987) wurde die Nestaggregation fast
an jedem Tag mit geeignetem Wetter beobachtet, die Häufigkeit der verschiedenen Verhaltensweisen zusammengestellt (Abbildungen 9a, b) und die Abhängigkeit von den
Wetterbedingungen diskutiert.
Sphecodes pellucidus SMITH,1845 (Apoidea, Halictidae)
und Leucophora obtusa (ZETTERSTEDT, 1838) (Diptera, An378


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thomyiidae) sind
IMHOFF, 1866.

Nestparasiten

von Andrena nycthemera

1. Introduction
Due to their great importance for pollination, feral
bees are regarded as meaningful for wildlife conservation and are studied increasingly. In the last years,
detailed investigations on several species of bees were
published (e.g. BELLMANN 19Sl, HAESELER 1982, HOHMANN
1978, MEYER-HOLZAPFEL 1984, GEBHARD & RÖHR 1987). Furthermore, a comprehensive monograph on the biology of
the German species of feral bees has been published
(WESTRICH 1989 a, b).
One of the largest genera of bees in Europe is Andrena
with more than one hundred species in Germany (WESTRICH
1984, WARNCKE 1986). There are several short notes on
the biology of different species of Andrena and many remarks in the faunistic literature. Nevertheless the behavior of most species is still unknown. The ethology of
A. nycthemera which is investigated in the present study
is virtually unknown. üp to now there were only few remarks in faunistic publications (PEUS 1926, STOECKERT
1954, KOCOUREK 1966, WESTRICH 1989 b). In most cases only the occurence of the species was recorded and its rarity pointed out (e.g. HAMANN & KOLLER 1965, WESTRICH
1985, DYLEWSKA 1987).
Apart from a detailed description of behavioral patterns, this study is focussed on two points: First, by
the continuous observation of a nest aggregation throughout a whole season we evaluated the seasonal history in
great detail. Secondly, we gained axact data on the life
of individual bees with the aid of individual labelling
of bees. Part of this material was presented in a preliminary form as an abstract (KLINKSIK & SCHÖNITZER 1988).
2. Methods
2.1 Time and duration of investigation
The nest aggregation was visited in the years 1983 to
1988, and in 1990. Most thoroughly it was observed during the season of 1984 (on 25 days) and 1987 (on 37

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days). In the latter year, the colony was visited almost
all days with good weather between March 24th and May
14th. On 12 days, the nesting site was not visited, but
on most of these days (except three) the weather was so
bad, with low temperatures, rain or even snow, that
hardly any bees had been active anyway. The observation
time on different days depended on the weather. It was
usually about 2 to 4 hours in the beginning and at the
end of the season and 6 to 8 hours in the middle of the
season, sometimes up to 10 hours (altogether more than
200 hours in 19&7).
All Statements of time mentioned in the text are given
in Central European Time (not summer time).
2.2 Observational techniques
For the most part, observations could be carried out
with the naked eye. The bees do not seem to be disturbed
by the presence of a quiet human observer. Of Special
help was a monocular field glass (8 x 20) which could be
focussed as near as 0.8 m distance. With this field
glass it was possible to scrutinize a rather large observation area without moving around.
The behavioral patterns of the bees were recorded with
a 16 mm film-camera (24 pictures/s, Kodachrome II) in
1987 and in 1984 with a black and white video camera.
The films were analyzed by Single frames.
Individual bees were labelled with small dots in five
different colors (shellack), according to the code system of v.FRISCH (1923). In 1987, about 150 individuals,

108 of them females, were labelled. 65 (i.e. more than
half) of the labelled females were seen again after labelling, 31 of them on 5 or more days. 17 of these females could be observed for two weeks or longer and 3
even for three weeks or longer. If an observation refers
to a labelled bee, the individual number is stated
throughout the text in brackets with "F" for females and
"M" for males, respectively.
The entrance of nests were marked with small rods of
aluminum (ca. 15 cm long, 3 mm diameter), labelled with
a pen. They were always stuck into sand about 3 cm east
of the exact entrance.
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2.3 Meteorological data
The elementary meteorological data (temperature of the
air and of the soil, cloud formation etc.) were recorded
each day we visited the nesting site. In addition we
used meteorological data of the weather service (Deutscher Wetterdienst) whose observation Station at Dachau
and Oberschleissheim is located about 3 or 6 km, respectively, from the nesting site.
3. The nesting site
3.1 Description of the nesting site
The nest aggregation is located near the village of
Hebertshausen, close to Dachau, about 25 km north of the
center of Munich. It lies in a sandy slope of about 25 x
120 m, about 475 to 485 m above sea level (Fig.l). The
slope has an inclination of about 40 to 45 degrees with
little steps and edges in some places. It faces the
south almost entirely, and at its eastern and western

side the ridge is protruding some meters, causing a
somewhat amphitheatrical shape. Furthermore at the sides
(predominantly at the western side) large trees shelter
the area from wind, an important fact for the microclimate.
The Vegetation of the slope is Mesobrometum, with Bra~
chypodiwn pinnatum s.str. in the western part and B. ru~
pestre and Bromus erectus in the eastern part. At the
foot of the slope (southward), the Vegetation of the
ground is Artemisietea. A small beaten path passes
through the nesting area.
The soil mainly consists of sand. The mean values of
particle size in three probes from the nesting area are
(right below the topsoil): 5 % clay, 20.6 % silt, and
74.3 % sand, of which 28 % is very fine sand (0.06 - 0.1
mm), 37 % fine sand (0.1 - 0.2 mm), 8.3 % sand (0.2 0.6 mm) and 1 % coarse sand. In the topsoil 8 % clay, 19
% silt and 73 % sand (24 %, 32 %, 16 %, 1 %, respectively) were found. At one place where searching flights
of females (paragraph 4-3-1) were quite frequent, but
rather few nests were built, the relative amount of clay
(14 %) and silt (40 %) was larger (sand 46 %).
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Two smaller parts within this area where the nesting
density is extremely high were observed with Special
attention and within these areas the nests were labelled.
- An area of about 4 m 2 in the lower third of the slope
(a sandy area, Mesobrometum with Bromus erectus, Festuca

Fig.l: Nesting site of Andrena nyothemera
southern Germany, facing towards west.
382

near

Dachau,


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ovina, Arrhenatherum elatius) was mainly scrutinized in
in 1984.
- The other area of Special attention is a sandy pit
with an inclination of about 5 degrees and its rim. It
Covers a total area of about 7-5 ni2 (Fig.2). About 50 %
of the sandy pit is covered by Vegetation (Arrhenatheretum elatioris). Arrhenatherum elatius and Rubus caesius
are in the majority, further plants found in this area
are Carex hirta, Festuca ovina agg.,Poa pratensis s.str.,
Anthyllis vulneraria, Ononis repens, Melilotus albus,Vicia cracca, AchiViea millefolium, Hypochoeris radicata,
Silene vulgaris, Taraxacum officinalis, Daucus carota,
Stenactis annua, Equisetum arvense, Cirsium arvense,0riganum vulgäre, Galium album, Plantago lanceolata, Cornus
sanguinea juv. This area was mainly scrutinized in 1987•
Willows, the food plant for A. nycthemera, are present
immediately adjacent to the nesting area (Salix caprea)
as well as in the near vicinity (50 to some 100 meters
distant). Within a distance of about 600 meters there is
the narrow lowland forest of the river Amper.
Besides A. nycthemera there are many other species of
bees to be found at this site. Remarkable is the occurrence of Rophites canus (several individuals caught by

Dr.K.WARNCKE, July/29/l986), a very rare species that
lives exclusively in extremely warm habitats (cf.STOECKHERT 1933, 1954; WARNCKE 1986; WESTRICH 1989). Conspicuous are hundreds of nests of A. vaga.
3.2 Preferred nest places and number of nests
The female bees prefer sandy places with scanty Vegetation for nesting. They build their nests as well into
the flat soil as into small vertical slopes or under
little overhangs (cf. Fig.2). Thus some of the entrances
of nests may seldom be in the direct sunshine. Often the
entrance is partially hidden beneath dry leaves or a
tuft of dry grass. The females seem to prefer rugged
soil to smooth horizontal places. In two cases it was
observed that a female apparently lived in a nest which
had been used before by another female. Since we did not
observe the bees within the nest itself, we can only
State that different bees used the same entrance.We nev383


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er observed, however, that two or more females used one
nest at the same time.
In the sand pit at its rim we labelled 99 nests of A.
nycthemera (plus 10 nests of A. vaga). It should be noted, however, that at least 13 of these were second or
third nests of the same female. The distribution of the
nests was far from being homogenous: Some parts of this
area were rather free of nests and others were crowded.
Several nests were very close to each other (about 5-10
cm), in one case (F44 and F04) the distance between the
entrances of the nests was only about 1-2 cm. In 19&4
within about 1 m2 of another place 12 nests of A.nyothemera were labelled (plus 40 nests of A. vaga and one of
A. fulva). We estimate from this a total number of about

500 to 1.000 nests in the whole nesting aggregation. We

Fig.2: Sand pit in which Andrena nyothemera was observed
with Special attention in 1987- In this area about one
hundred nests of Andrena nyothemera were recorded.Meterstick in the foreground: 1 m.

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could not notice any alternation in population siae during the years.
4. Behavioral pattern
4.1 Behavior of males
4.1.1 Crawling and inspecting holes
One of the most common behavioral patterns of males is
crawling on the soil, especially when it is cold or
cloudy. Frequently the antennae are bent downwards, almost touching the sand, or they are moved up and down.
Quite often a male will search again and again at one
particular spot. If it is warm and the sun is shining,
the crawling is hectic and often interrupted by Short
flights (typically ca. 25 cm - 1 m ) . The males inspect
every little hole. Either they just poke their head into
the entrance for a moment or they enter completely and
come out backwards after some seconds. Sometimes they
stay in a hole for 5 minutes or even longer.In such cases they emerge from the hole with the head first and
almost always clean their antennae immediately after coming out of the hole. It also happens, that a male emerges from a hole, crawls around a little bit (about 10cm)
and then reenters the hole.
4.1.2 Digging
While crawling on the soil, the males often dig pits

in the sand which are about l/2 cm in diameter and about
1/4 cm deep. The sand is scraped off with the mandibles
and the anterior legs. The anterior legs - and sometimes
the middle legs - push the loose sand bachwards.The posterior legs stabilize the bee. Quite often different
males dig at exactly the same place one after another or
simultaneously side by side.
4.1-3 Aggressive behavior
Males aften compete for females to copulate with (cf.
Paragraph 4>2) or for a place to dig. Usually one male
seems to push the other one away. Sometimes, however,
real fights occur in the course of which the bees bite
one another with the mandibles, then they (both) roll

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about. In one case (Apr/4/87) three males were competing
for a hole and biting each other several times with the
mandibles until one of them, after about 10 minutes,
succeeded in chasing the other ones away.
4.1.4 Patrolling flights and sitting in the sun
Usually the males of A. nycthemera fly very close to
the soil (about 2 to 10 cm) over the nesting area. They
do zigzag routes or, less common, large loops. In the
first part of the season the males tend to fly in smaller loops than later in the year. Alternating with their
flights the males frequently stop to crawl around, dig,
or sit down on leaves or stalks. For resting, they prefer sunny places like patches of dry sand or pale leaves,
especially in the first part of the season, when it is

rather cold but sunny. At low temperatures the males
tend to fly close to the soil and mostly perform short
flights of only some meters. When it gets warmer they
fly higher, faster, and perform longer routes. They may
fly up to an altitude of about 1 m.
4.1.5 Territorial behavior
Although the males do not have fixed patrolling routes
they are seen again and again in the same area. Very
often a male keeps crawling around and digging within a
Square meter for about half an hour and may be seen at
the same place the next day again. In one case, for example, 10 males were labelled within a sandy area of
about 5 m 2 , some meters away f rom the main observation
area. During the next four days these males were seen
again always at their original place or at a maximal
distance of 5 m from it. Another male (MIO) was for example labelled on March 29th,1987 and seen again at the
same place several times on 6 consecutive days. In the
contrary to descriptions in Andrena chrysosceles (HAAS,
I960) we could not observe any behavior which may be interpreted as scent labelling.
4.1.6 Spending the night
The males spend the night in the soil. In the evening
they can be observed to enter holes, the majority of
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which are probably nest entrances. Some males can be
seen to burrow themselves at an existing nest entrance.
In the early morning male bees can be found within the
holes about 1 to 3 cm below the surface.

4.1.7 Pouncing
A most conspicuous behavior of males is that they
pounce on females, trying to mate. They pounce on any
female no matter whether it is sitting, crawling or digging. They even pursue females in the flight or follow
them into their nests. They also try to copulate with
dead females or ones with pollen in their flocculus.
Males also often pounce on other males or other dark objects like buds, small leaves or pieces of wood. Usually
they realize their error quickly (less than 1 second)
and leave.
4.2 Mating
Altogether we could observe nine copulations, three of
which were observed in 1983 on one day (Mar/21), shortly
after another. All others were seen in 19S73 each on
different days (Fig.9a). Two copulations were observed
from the beginning, these lasted about 2 or 4 minutes,
respectively. In two cases the copula obviously had begun within a burrow, since the female emerged from a
nest entrance in copula with a male on her back. In one
case (Apr/6/1987) the male had entered the hole just
five seconds before.
The head of both partners point in the same direction,
the male is on top of the female. First the male clasps
thorax and abdomen of the female with its legs, then it
bends the abdomen downwards to insert its genitals. Some
males then raise their body vertically keeping contact
to the female only via the genitals. Often the male produces some pulses of a buzzing sound which seems to be
evoked by moving the wings and is accompanied by rhythmic contractions of the legs. Sometimes the metasoma is
contracted a couple of times in a pumping motion. The
female usually does not move during the copula, only in
one case its posterior legs were wiping the abdomen. In
one case (Mar/27/1987) two males competed on the back of

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one female, the female first tried to repulse both males
but then copulated with one of them, and the other one
flew away.
Usually the females terminated the copula by scraping
the male down. Afterwards they rested for about a minute
before they flew or crawled away. In one case the female
was just digging before mating and it continued digging
immediately afterwards. In three cases the males were
captured after mating to see whether they would survive
(cf. MICHENER and RETTENMEIER 1956). They all survived
for 48 hours and were then released.
4-3- Behavior of the females
4-3-1 Searching for a nest site, searching flights
Especially
in the beginning of the season a great
number of females of A. nycthemeva can be observed to
perform a typical kind of flight: They fly in many loops
or zigzags over the nesting area, mostly at an altitude
of about l/2 to 1 m over the ground, alternating with
flight courses very close to the ground. Certain parts
of the area are frequented more than others, but these
places are not always identical to those where the nesting
density is especially high. In one of these preferred places the soil was analyzed and found to contain
a large percentage of clay and silt (cf. paragraph 3-1)•

The flights may often be interrupted by crawling around
(often with lowered antennae) and a little scraping or
digging. In many cases a female will inspect a certain
spot (e.g. some 10 x 10 cm) for about 10 minutes or more,
alternating with short flights.

Fig.3 (p.389): A female Andrena nycthemeva repulsing a
pouncing male, a and b: the male is approaching in flight.
c, d: the female is repulsing the male with the right
posterior leg (arrows). e: the male flies away (oppen
arrow). f: the female has put its right posterior leg on
the ground again. Single frames from a film sequence
(0.5 s), with the following numbers in the film: 13-613,
618, 621, 624, 625, 630.

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4.3-2 Repulse the pouncing males.
Very often the males pounce onto the females (paragraph 4-l-7)j these, however, are usually not receptive
and repulse the males in most cases (Fig.3)« Any type of
female behavior may be interrupted by this. The females
crawl away, often beneath grass blades or leaves to

strip off the males which try to mount them. Often the
females roll over the sand very quickly with the males
on top. Sometimes the females use the middle and/or
posterior legs to keep the males away (Fig.3)- It seems
that ev.en during the flight females accelerate or perform sharp turns to escape pursuing males.
Sometimes unsuccessful mating attempts are initiated
within the nests. For example, it was observed (Apr/6/
1987, 11:30) that a male entered a hole and some seconds
later a female came out with the male on its back. The
female tried to scrape it off with the posterior and
middle legs, but the male clinged to the back with all
legs. It took almost two minutes untill the female got
rid of the obtrusive male.
4.3-3 Spending the night
Before the females have constructed their own nests
they, like the males, spend the night in holes which
probably are mostly old nests. Sometimes, however, in
the eveneing they burrow themselves into the sand at a
Fig.4 (p.39l): Female Andrena nycthemera scraping the
left antenna. a: the right foreleg is lifted over the
antenna; arrow: tibio-tarsal Joint,with the antenna cleaner beneath; arrowhead: lowered left antenna. b: the flagellum is inserted into the antenna cleaner. c: beginning of the stroke, arrow points to the praetarsus of
the left (cleaning) leg, beneath it is the left middle
leg. d and e: the flagellum is cleaned, the part of the
flagellum which is not yet cleaned is indicated by an
arrow. Note that it is bent upwards. f: end of the cleaning stroke. Single frames from a film sequence (0.6 s ) ,
with the following numbers in the film: 12.541, 545? 546,
548, 550, 555390


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place which is quite obviously not suitable for a nest,
like i.e. a lump of earth.
4.3-4 Grooming
Any other behavior can be interrupted by self-cleaning
behavior. Since Single cleaning procedures are quite
fast, they were usually not recorded. We did not find
any difference to the grooming behavior previously described for other species of Andrena (JANDER 1976).
By analyzing film sequences of the cleaning of the antenna it is seen that the first part of the cleaning
stroke (when the basal part of the antenna is cleaned)
is clearly faster than the second half of the stroke,
cleaning the distal part of the flagellum (cf. Fig.4:
the time elapsed between frame d and e is 2/24 s, the
time between e and f is 5/24 s). Similar to the Situation in the honeybee the distal part of the flagellum is
bent upwards, and the tibio-tarsal Joint is bent strongly (SCHÖNITZER and RENNER 1984).
4.3-5 Digging and building the nests
Digging is accomplished by biting the sand with the
mandibles to loosen it (Fig.5)« With the anterior legs
the bees scrape the sand out of the depression and shove
it under the body. Then the middle legs push the sand
further backwards. The anterior legs scrape in a much
faster rhythm than the middle legs. The posterior legs
stay remarkably immobile, they are sometimes stretched
out perpendicularly from the body. When the animal is
already partly dug in, it turns the whole body, usually

performing one or two complete turns counterclockwise.
In few cases they do half a turn to the right, then to
the left etc. The loose sand is pushed out of the burrow
Fig.5 (p-393)"- Female digging into its nest with pollen
on its posterior legs. The two patches on the thorax are
labeis. Note the lowered antennae in a and b. a is somewhat differently trimmed than the following fotos.Arrows
point to grains of sand just being pushed out. Single
frames from a film sequence (4»8 s), with the following
numbers in the fim: 15.O63, 071, 074, H O , 113, 178.
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with the abdomen arid/or legs (Fig.5)Once the bees have completely disappeared in the soil
their digging activity can be seen by the movement of
the soil and the periodical discharge of sand. The digging is alternating with intervals of no visible activity. The sand is piled up and forms little tumuli at the
entrances, these may be up to about 3 cm high and have a
maximum diameter of about 10 cm. Usually they are washed
away by the next rain.
During that time the females rarely leave their nests
but dig for about 2 to 4 days. Afterwards they supply
the nest with pollen. It seems that the females prefer
days with bad weather to work in the nests. It happened
that bees dug for one day only before bringing pollen

into the new nest (F56, Apr/14/87; F84, Apr/lO/87), on
the other hand, a bee (F93) had been living in a nest
for 5 days before it brought the first load of pollen in.
Sometimes a female continues digging within its nest into which it has already brought pollen, and it supplies
the same nest with pollen later (e.g. F23, F17). Or a
bee digs at a new site for one day, but then takes care
of the former nest again (F01, 24.4-87)•
4-3»6 Duration of living in a nest
Usually the females spend about 5 to 10 days in the
same nest, i.e. were seen to emerge from it or enter it
(with or without pollen loads). This is recorded in 23
cases, but lack of observations possibly biases these
results, thus the real times may be longer in some cases.
Some bees have been observed to live in a certain nest
for a remarkably longer time. For example the bee F03
was altogether seen on 11 days out of a total of 26 days.
The first and the last time it was seen (Apr/6, May/l/
87), it could not be related to a nest, but at 9 days
(between Apr/9 and Apr/28) it was seen to enter and/or
emerge from the same nest. Thus it may be concluded that
it used this nest throughout the whole period of three
weeks. Other bees were recorded to live in one nest for
11, 12, 13, 14> 15 (two cases), 17, and 18 days respectively.
On the other hand, it should be noted that some bees

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spend only few days in the same nest. For example, the
female F44 lived in three of its nests for only one or
two days (possibly 3 days, due to lack of observation).
4.3.7 Number of nests per bee
Of those 17 bees, which could be observed over a period
of more than two weeks, 10 were noticed to bring pollen
into one nest only; one bee (F03, cf.above) was observed
to inhabit one nest for about three weeks. Four bees
provisioned two nests, two bees three nests and one (F
44) even four nests.
The bee F44 inhabited 6 nests, the maximum number in
our investigation, and it brought pollen into four of
them. All those nests were within the above described
sandy pit.
4.3.8 Emerging from the nest
When females emerge from the nest, they come out with
the head first and usually remain in the entrance for
some time (about 5 seconds or longer, cf. next paragraph). Typically they walk some centimeters southwards
(downhill) and then take off. The bees which nest under
an overhang (thus the entrance ia always in the shadow)
usually crawl to a sunny place and stay there before
they take off. Sometimes females fly directly from the
entrance to a blade of grass or a pale leaf and stay
there for about l/2 to 2 minutes before leaving the area.
Predominantly at the end of the season, when the Vegetation has grown, bees frequently climb the top of a stalk
or blade of grass and start from there.
4.3.9 Sitting in the entrance
Especially if the weather is too cold or too cloudy to
fly (cf. paragraph 5-4), the females often sit in their
nest entrances for quite some time. They often remain

quite immobile for several minutes with the head in the
nest entrance and only the antennae projecting outside.
At times the majority of females in the nest aggregation
may be seen sitting and waiting in the entrance. When
the temperature is rising the animals leave their nests;
if it stays cloudy and cool they return into the burrows

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after some time. This behavior, however,is not seen during the first part of the season (i.e. before the females have adopted their own nests).
4-3-10 Closing the nest entrance
Before the females take off from their nests (e.g. for
collecting food) they usually close the entrance of
their nests. With the posterior part of their abdomen
still in the entrance, the females shove loose sand under their body with their anterior legs and push it into
the hole with the middle legs. Sometimes females turn
around afterwards and brush some sand over the entrance
with the anterior legs. In some cases a small depression
is still visible indicating that the bee has left the
nest and closed it from the outside. At times, however,
females do not close their nest entrance after emerging.
This is especially frequent for bees nesting in a vertical slope. If the closing procedure is interrupted, for
example when the bee is falling and rolling down the
slope, the bee does not continue to close the entrance,
but take off.
Usually the bees also close their nest entrance when
they return into the nest. They first dig themselves into the loose sand and then push the sand backwards with

the abdomen. Sometimes, however, in about 5-10% of the
cases, females enter their nests and leave the entrance
unsealed.
4.3-11 Orientation flights
After take off the females frequently perform some
loops which obviously are orientation flights. Immediately after take off they turn around in the air to face
the entrance of the nest. In this position they have the
same view as at the landing approach. For a moment they
stay in the air (about 5 to 10 cm high), then they fly
some turns which are about semicircular, first narrow
and then larger, until the bees leave the nesting site
with a large loop. Similar orientation loops are seen
before landing.
Especially during the first days in a new nest, the
females perform long and exhaustive orientation flights.
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After some days they only do few loops. If, however, the
immediate surroundings of the nest have been altered (by
external events, e.g. someone stepping on the nest entrance) the bees once again perform extensive orientation flights.
4-3«12 Returning into the nest
Especially in the beginning of the season the females
do not always succeed in finding their nests at once.
When they come back from collecting pollen or from feeding they usually fly in loops or zigzags for some minutes at an altitude of about 1 meter. Then they go deeper
(some 5 to 10 cm) and scan a small area (about 30 x 30
cm). Sometimes they stay in the air for a moment before
they land and crawl to the entrance of the nest. After

some days the bees find their nest entrances usually
within a few seconds after a rudimentary orientation
loop. They land close to the entrance (within a few centimeters) and dig in very fast.
The females nesting in a vertical slope or under an
overhang usually do not fly directly to the entrance,
but land further down on the flat ground. From there
they crawl up the slope clumsily with buzzing wings.
Often, this is followed by tumbling and rolling downwards over the loose sand then again crawling up and so
on. Generally we had the impression that the females on
the vertical slope had more problems finding the entrances of their nests than those on the flat ground. Sometimes, however, they fly directly to the entrance hole
and enter it very quickly.
The returning to the nest by the females seems to be
somehow synchronized. Quite often there is not a single
bee to be seen at the nesting site for several minutes
and then suddenly several bees return to their nests at
the same time. This is especially frequent at the late
afternoon in the beginning of the season.
4.3»13 Searching the entrance
The female bees find the appropriate site of their
nests right away, but often they have problems in finding its entrance. So they crawl around and search all
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over the sand. Lowering the head they bend the antennae
downwards, touching the sand with them. For a time they
will dig, sometimes burrowing themselves in the soil and
staying there for up to about 10 minutes. Occasionally
they take off again, leave the nesting area, stay away

up to half an hour and return again with a new orientation flight.
One female (F07, Apr/17/1987) was observed twice to
search its nest entrance digging at a wrong place although the entrance of its nest was open and undisturbed.
Such a search may even take some hours. One bee (Fl83
Apr/18/1987) was seen to search again and again at the
same place almost for a whole day (about 6 hours) and
for about another two hours on the following day. Then
it began to rain and the bee was not seen again any more.
Nevertheless it should be noted that the great majority of females find their nest within only a few seconds.
4«3-14 Provisioning
The number of pollen loads per day which the females
bring into their nests obviously depends very much on
the weather. There are many days, especially at the beginning of the season, when no more than one load of
pollen is brought in by the females. In the afternoon of
such days/ several females can be seen within a short
time (about 15 minutes) to return into the nests with
pollen. Early in the season, even if the weather is good,
most females bring only one or two loads of pollen.Later
at the peak of the season, the Standard is three
pollen loads a day, ranging from one to as many as six
loads. Towards the end of the season the number of pollen loads goes back to one or two again (Fig.6).
The average duration of pollen collecting flights is
about 30 minutes, but it may be as long as 100 minutes
(5c = 38.45 min; 0 = 17-7; n = 60; Fig.7). After returning with pollen, the bees usually stay in the nest for
about 20 to 75 minutes, sometimes up to 145 minutes (x =
54-9; o = 77-0; n = 72; Fig.8).
In many cases the females also leave the nesting area
for some time and return to their nest without any pollen. These flights are interpreted as seif-provisioning

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flights and usually last for 10 to 65 minutes with a
mean value of about 30 minutes (x = 29.25; o = 17-25;
n = 12). Towards the end of the season the duration of
these flights increases (95, 105, and twice 150 min respectively, Apr/23 and Apr/24/1987).
4«3-15 Aggressive behavior
In seven cases agressive behavior of females has been
number of females

12r
10

m
2

3
4
5
pollen flights per day

• 1 Apr. 9 and 11

Apr. 17 and 18

H H Apr. 23 and 25

Fig.6: Number of pollen flights per day: at the beginning, in the middle and at the end of the season. Each

set of columns counted at two days with good weather and
an observation of at least 8 hours.

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observed. In all except one case (see below) the reason
was clearly competition for a nest caused by a female
which occupied - or tried to occupy - the nest of another
female.
In two cases the confrontation was rather short (about
1 to 2 minutes): A female returned to her nest with pollen, found the nest occupied by another female, nevertheless entered the nest and pushed the intruder out.
After the fight the winner stayed in the entrance of the
number of females
20 -i

15-

10-

10

20

30

40

50

60

70

80

90

100

min
Fig.7: Duration of pollen collecting
nycthemera.
400

flights of Andvena


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nest, sticking the head out of it for at least 20 minutes as if now guarding the nest.
In another case the bee who returned to its nest (F94,
Apr/8/87) tried to enter it several times for about 10
minutes but was pushed out again and again. Finally it
succeeded to enter the nest and about 4 minutes later
the entruder (F50) came out.Then this bee (F50) was repeatedly hindered from entering the nest again.Some minutes later it began to dig somewhere eise, first at a
number of females
10r

20

40

60

80

100

120

140

min
Fig.8: Length of time spent in the nest by females of
Andrena nycthemera after having brought pollen into it.
401